Angel is mostly known for that. It also follows the same timeline. So you can watch both shows back to back. As much as I enjoy this show, I still think Buffy the Vampire Slayer is better. And I don't see what makes it more mature, either. It even happened to be the opposite for me, especially during the first season. And some stand alone episodes, as well as the Pylea's arc, that made the Angel series, look sillier and ultimately targeted to a much younger audience. I don't mind it, after all I'm a huge fan of WB's old series. But I hate how some fans made it out to be something it's not, and the cartoonish comic reliefs, didn't help make its case either. To me, the series also suffered from season three serialized plot and the new character it was focused on. It was too melodramatic for its own good. I'd say, watch season one for the cool monsters of the week, and crossover episodes. Season two, for Darla's return. Season three, out of curiosity. Season four, for how crazy and over the top it can get. And season five, for completely exceeding your expectations of a finale season.
Angel
(1999–2004)
First spin off series, that is almost as good as original.
3 May 2021
Warning: Spoilers